metamorphic
Gneiss
Gneiss (High-grade metamorphic rock)
Hardness: 6-7 on Mohs scale; Color: Banded pink, white, and dark grey/black; Luster: Dull to vitreous; Crystal Structure: Coarse-grained, foliated with distinct banding (gneissose banding); Cleavage: Minimal (breaks irregularly).
- Hardness
- 6-7 on Mohs scale
- Color
- Banded pink, white, and dark grey/black
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded pink, white, and dark grey/black; Luster: Dull to vitreous; Crystal Structure: Coarse-grained, foliated with distinct banding (gneissose banding); Cleavage: Minimal (breaks irregularly).
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks (like shale). It is subjected to extreme temperatures and pressures, causing the minerals to segregate into distinct layers.
Uses & applications
Primarily used in construction as crushed stone for road bases, and when polished, as architectural stone, countertops, and decorative flooring.
Geological facts
Gneiss is some of the oldest rock on the planet; the Acasta Gneiss in Canada is approximately 4 billion years old. The banding occurs due to a process called metamorphic differentiation.
Field identification & locations
Identify in the field by looking for 'gneissose banding'—distinct light and dark layers of minerals that are often folded or wavy. Found in the cores of mountain ranges and ancient continental shields worldwide.
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